Exposed fiberglass underlayment and granular loss were noted at a recent home inspection in Oak Harbor on Whidbey Island.  This was likely caused by someone pressure washing moss off the roof system.  The issue is that moss has rhizomes, or roots, that imbed into the roof coverings.  When mechanically removed, they take with them the asphalt and granules from the roof coverings, exposing the underlying fiberglass mat, and rendering the roof system compromised.  I strongly recommend against the practice of pressure washing as I’ve seen too many roofs destroyed by this practice.  Treating your roof on a semi-annual basis with zinc granules is advised to help prevent moss growth.  If the growth is pronounced, treatment more often will help speed the process, but it will take time.  Personally, I like a perfect roof, so I treat my new roof four (4) times annually with zinc granules readily available at all hardware stores.  Some homeowners prefer Tide with bleach, others say baking soda works.  Treat, don’t pressure wash!